What are some good tactics for spells that are not obvious? Yes, many will be situational.

Examples:

(1) If you have flight (owlin, faerie, winged tiefling, etc), fly 15 feet above an enemy and cast lightning lure. On failed STR save, enemy is pulled 10 feet up, takes lightning damage and then 10 feet fall damage and is prone (melee against him is at adavantage and his at disadv until he gets up).

(2) If you are a sorcerer with a bloodwell vial, wither and bloom will trigger you getting sorcery points back.

Maelstrom used with any other AoE is really fun. My warlock uses Hunger of Hadar and the druid uses Maelstrom to blind, freeze, bludgeon, acidic slime, and continually drag our opponents to the epicenter over difficult terrain laden area.

Oh, conjure minor elementals (8 steam mephits) works well to drag enemies into a wall of fire (someone else cast) if you can roll well enough. You do get 8 chances. :)

Transmute Rock is the ultimate anti-fortress spell, you can transform a 40 ft cube of stone wall into flowing mud leaving a 40ft hole in the wall as a single action, potentially collapsing it underneath the defenders atop the wall. Transmute Rock also can combo with any kind of wall spell which you can cast on top of the enemies sinking into the mud to completely cripple them.

Warding Wind makes you immune to many monster charms.

Blur + blink can be an awesome combo, especially for hexlocks. And yes, I know that blink doesn't always work; I personally don't care and feel like the uncertainty is part of the fun. Regardless, the combination can give a PC in melee effective tactical and defensive advantages.

Gust of Wind is one of the best ways to help your allies escape any monster with 5ft reach that grapples.

A Hasted Moon Druid-8 can deal 32d6 as a Quetzalcoatlus by Grappling then flying 320ft upwards (80ft flyx2 (haste) x2 (hasted action)) and dropping them

Haste + Polymorph allows you move 100ft and deal 86 damage per round as a T-rex

Saying Blink doesn't always work is like saying that sometimes people hit you even if you Blur.

Create Bonfire and Booming Blade

Quote from Agilemind >>

Gust of Wind is one of the best ways to help your allies escape any monster with 5ft reach that grapples.

A Hasted Moon Druid-8 can deal 32d6 as a Quetzalcoatlus by Grappling then flying 320ft upwards (80ft flyx2 (haste) x2 (hasted action)) and dropping them

Haste + Polymorph allows you move 100ft and deal 86 damage per round as a T-rex

 

 Doesn't quite work like that.

Quote from Wysperra >>

Quote from Agilemind >>

Gust of Wind is one of the best ways to help your allies escape any monster with 5ft reach that grapples.

A Hasted Moon Druid-8 can deal 32d6 as a Quetzalcoatlus by Grappling then flying 320ft upwards (80ft flyx2 (haste) x2 (hasted action)) and dropping them

Haste + Polymorph allows you move 100ft and deal 86 damage per round as a T-rex

 

 Doesn't quite work like that.

 Quetzalcoatlus is a Huge creature so it can move it's full speed while grappling a Medium sized creature.

What I meant was maximum falling damage is 20d6.

Quote from Mog_Dracov >>

Saying Blink doesn't always work is like saying that sometimes people hit you even if you Blur.

 
I was trying to preempt the types of players who want things to work maximally all the time.

Thornwhip would work instead of lightning lure. :)

Quote from Foodgeek >>

What are some good tactics for spells that are not obvious? Yes, many will be situational.

Examples:

(1) If you have flight (owlin, faerie, winged tiefling, etc), fly 15 feet above an enemy and cast lightning lure. On failed STR save, enemy is pulled 10 feet up, takes lightning damage and then 10 feet fall damage and is prone (melee against him is at adavantage and his at disadv until he gets up).

(2) If you are a sorcerer with a bloodwell vial, wither and bloom will trigger you getting sorcery points back.

 Not sure if these are not well known, but..

• Prismatic wall -> Reverse Gravity
• Polymorph -> Power Word Kill (I HATE this one)
• If you have fire immunity or a way to fully negate fire damage, casting heat metal on your weapon will cause it to do an extra 2d8 free fire damage
• Casting haste on an enemy and then dropping it instantly to so they get the lag time
• Conjure Woodland Creatures (if you can choose your summons) 8 pixies and they all cast polymorph. Also, when you break conc on the Conjure spell, they're sent back to the FeyWild, but the pixies don't lose their concentration on polymorph. 
• Banishment on yourself as a free Plane Shift if you're off the Material Plane
• Not really a spell exploit, but you can use Sharpshooter on Magic Stone if you're using a sling to throw it. I made a magic stone build with around 200 DPR.
• Forcecage -> Sickening Radiance
• Hold Person -> Disintegrate 
• Find Familiar Inflict Wounds shenanigans, works well if you have sorcery points. Or contagion.
• Standard Spiritual Weapon + Spirit Guardians on a cleric
• Ceremony on a commoner, polymorph to carry them around for +2 AC, then kill them when the time limit ends. Can repeat this several times. (Not morally acceptable in most cases)
• Spike Growth, ranged bludgeoning weapon, and crusher. Or repelling blast. Vortex Warp works too.
• Vortex Warp as a great ally positioning spell
• Cloud of Daggers/Wall of Fire and Sentinel + War Caster. Carry around a polearm with reach and/or play bugbear, and you can just keep reducing their speed to 0 anytime they try to get close, while they spent their turns in the area effect.
• With Counterspell, technically Jack of All trades applies. Along with Knowledge of a Past Life, Guidance, and Bless. 

I'm not a frequent spellcaster player, so I'm mostly theorizing.

Not sure if it's lesser known but we have done Plant growth and Sickening radiance

Plant growth is an absolutely terrific spell for enemy casters/shamans to use against the party when in wilderness environs. It's especially effective if the enemy has ranged attack capabilities.

• true seeing or devil sight, mind blank and maddening darkness.  It gives you an area of darkness and damage that is safe for you  but not enemies. I often have bosses use it.
• otto's irresistible dance  is good to keep targets in areas of effect particularly if  escaping that spell also requires an action like [Tooltip Not Found] or black tentacles
• the necromancy wizard ability inured to death lets you cast spells like create magen  and Create Homunculus without reducing your maximum hit points.
• The feat Eldritch Adept can  give you unlimited casting of silent image which allows you to use illusory reality at will as a level 14 or high illusion wizard. Thought it comes online late. 
• The feat Eldritch Adept can give you unlimited casting of mage armor which allows you to always recharge your ward 
• Arcane ward  from the abjuration wizard extends the hit points of armor of agathys which can be obtained from feats allowing you to do more damage with it.
• a psi crystal allows you to bypass most language requirements for spells and a ring of animal influence allows you to cast speak with animals these two items can allow your enchantment wizard to cast suggestion on most creatures 
• you can command tiny servant to use items which can allow you to activate items at distance from you. The extent of this is unclear but it could be any where from  using equipment like ball bearings to activating magic items like wands

Yeah, devil sight (eldritch initiate) through a feat works well with darkness through shadow sorcerer or shadow monk too (or shadow spell in general). Or take the blindsight feat (fighter initiate or whatever it's called) but it has less range.

For a Warlock Mental Prison followed up by Repelling Blast or similar capable of forcibly moving someone. Should give 15d10 plus the damage from Eldritch Blast

Spirit Shroud/Spirit Guardians plus anything that knocks prone. Once they stand up, their speed is zero, and if they don’t get up, they can’t escape the area. 

Vortex Warp is a cheap way to guarantee AoE retriggers. Illusory reality, Vortex Warp and Phantasmal Force allow you to farm fall damage by making a real and then not real platform for them to stand on. 

Not a spell combo per se, but using Shapechange into a polukranos if you have fire immunity allows you stack up heads (and therefore attacks and reactions) essentially forever. 

Summon greater demon doesn’t specify you can’t summon summoner variants, meaning if you just intend to cause chaos you can get double the demons per spell slot. 

You can cast forcecage around an echo knight fighter and make them immune to basically everything for an hour against big or non-teleporting enemies, while their echo still gets to run around and kill stuff.

Magic Missile. Anything that specifies ‘add to one damage roll of a spell’ applies to every single missile. You can get fifty-plus guaranteed damage out of a 1st level slot, with the right build.

Conjure Woodland Beings can turn an eighth-level party of eight into T-Rexes, or four martials into flying T-Rexes. 

Jim’s Magic Missile triggers bugbear surprise attack.